Steam Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS), also referred to as the ‘Huff and Puff’ process, are the two most commonly used methods for recovering bitumen and heavy viscous oils from subterranean formations. In a SAGD method a pair horizontal wells serve both as an injector well and a production well. The injector well and the production well extend downwardly from the surface of the earth into the oil bearing stratum and then extend horizontally through the stratum. The horizontal portion of the production well is placed at the base of the oil bearing stratum and the horizontal leg of the injector well is placed at an optimal distance above and parallel to the production well. Steam is injected continuously into the stratum through the top well. Upon entering the stratum, the steam releases it latent heat to the stratum, thereby heating and lowering the viscosity in the formation and improving the mobility of the oil. The heated oil with condensed steam flows down under gravity toward the bottom production well from which it is produced.
Cyclic steam stimulation or the ‘huff and puff’ process is another commonly used method for recovering heavy viscous oils or bitumen from the subterranean formation. This process involves injecting a predetermined volume of steam into a stratum through a single vertical well for a selected duration, stopping the injection of steam, permitting the stratum to soak and followed by producing oil through the same well. Unfortunately, CSS has a number of limitations because it is a single well process and the recovery zone is limited to the near well bore region. Most notably, the ability to improve recovery requires CSS to drill a large number of wells.
There are a variety of problems associated with the ‘SAGD’ and the ‘Huff and Puff’ processes. A commonly used criterion to establish the efficiency of a steam based oil recovery process is the steam oil ratio (SOR). SOR is the measure of barrels of steam needed to produce one barrel of oil. Both thermal and recovery efficiency (and by implication the economics) of the process is a function of SOR. A smaller SOR implies higher thermal efficiency and better economics. Both SAGD and CSS are thermally inefficient. A major drawback of the SAGD processes is that it requires large volumes of high quality (90-100% quality) steam to heat the reservoir and recover oil at economic rate. In thick, high quality reservoir the SAGD recovery can exceed 50% of oil from the steam contacted region at a SOR greater than 3.0, but in thinner reservoir with lower oil saturations, SORs are much higher.
Also SAGD oil recovery depends only on the available latent heat content of the steam (sensible heat has no value to the recovery process). Since the latent heat content of the steam decreases with increase in steam pressure the SGAD process becomes uneconomic if implemented at greater depth (reservoir pressure and consequently the steam injection pressure increases with depth). The use of surface generated steam as the heat source also limits the depth at which the process can be implemented due to conductive heat losses (degradation of steam quality) through the well bore.
Irrespective of reservoir quality, steam quality and oil saturation, CSS recovery method seldom exceeds 20% oil in place at SORs in excess of 5. Use of lower quality steam in the CSS process in reservoirs containing highly viscous oil is less effective in lowering the viscosity of the oil, limit the radius of invasion of the steam, lower the reservoir sweep and result in lower recovery due to slower rate of flow of higher viscosity oil during the production cycle. In CSS processes high heat efficiency would be promoted by lower pressure and high quality steam. Thus processes that promote high recovery of viscous oil at lower SOR are needed. Conventional steam injection processes such as SAGD and CSS produce SOR in the excess of 3.0.
There exists a need to provide an improved viscous oil recovery process that overcome the limitations of SAGD and the CSS process to provide profitable recovery of heavy oil/bitumen from thinner formations than currently permitted with conventional steam heating processes.